Inductively coupled plasma (ICP) process reactors generally form plasmas by inducing current in a process gas disposed within the process chamber via one or more inductive coils disposed outside of the process chamber. The inductive coils may be disposed externally and separated electrically from the chamber by, for example, a dielectric lid. When radio frequency (RF) current is fed to the inductive coils via an RF feed structure from an RF power supply, an inductively coupled plasma can be formed inside the chamber from an electric field generated by the inductive coils.
In some reactor designs, the reactor may be configured to have concentric inner and outer inductive coils. The inventors have discovered that additive electric field properties (due to destructive interference of the magnetic fields induced by the coils) between the inner and outer coils can result in non-uniformities in the electric field distribution of the plasma formed at the substrate level away from the coils. For example, due to etch rate non-uniformities caused by the non-uniform electric field distribution in the plasma, a substrate etched by such a plasma may result in a non-uniform etch pattern on the substrate, such as an M-shaped etch pattern, e.g., a center low and edge low etch surface with peaks between the center and edge. The inventor's have further observed that adjusting the power ratio between the inner and outer coils to control the severity of the non-uniformity is not sufficient to completely eliminate the non-uniformity. Moreover, in order to meet the critical dimension requirements of advanced device nodes, e.g., about 32 nm and below, the remaining etch pattern non-uniformities due to this phenomenon may need to be further reduced or eliminated.
Accordingly, the inventors have devised a plasma process apparatus to better control plasma processing non-uniformity.